1. Field of the Invention
The present invention generally relates to material inspection and, more particularly, to apparatus for inspection for surface cracks in ferromagnetic materials.
2. Description of the Prior Art
Ferromagnetic materials such as iron and steel are used in the construction of many structures. When employed in load bearing members of such structures, such materials are often subjected to bending moments. These bending moments are carried within the members generally as either tension or compression which will usually vary from point to point within the volume of the structural member and be maximized at the surface of the structural member.
During the fabrication of iron and steel structural shapes and plates, the iron or steel is typically brought to final dimensions by repetitively passing it through spaced, shaped rollers. This process is done at high temperatures to minimize strain hardening of the material. However, it is relatively common for some microscopic cracks to develop at the surface of the material.
Such surface cracks are particularly critical because it effectively reduces the cross-sectional area and transverse dimension of the structural member or plate. When tension is present at the surface containing the crack, the tensile forces are concentrated at the bottom of the crack. Although this is often of no concern due to safety factors in design which keep tensile forces below levels which would cause metal fatigue, or further propagation of the crack, it may be critical in other applications in which the amount of structural material must be minimized to save weight or for other reasons. In such applications, there is no alternative to inspection of the material for surface flaws. Such inspection is difficult because of the typically small size of such flaws and the potentially large area which must be inspected.
One inspection technique, however, which is effective and sensitive for such flaw detection is referred to as magnetic particle inspection. In this technique a strong magnetic field is made to propagate through the material in a direction generally parallel to the surface to be inspected. If the field is sufficiently strong and the material brought close to magnetic saturation, any crack oriented generally perpendicular to both the surface and the magnetic field causes a strong leakage field at the surface of the material at the location of the crack (e.g. discontinuity). This leakage field can attract fine iron particles and flaws may thus be easily detected upon visual inspection. The field within the material is generally established by passing a large AC electromagnet yoke across the material surface and thus a relatively large area of material surface can be rapidly inspected.
However, the strength of the magnetic field in the material is fairly critical to the success of this inspection method. If the flux density within the material is not sufficiently great (e.g. close to saturation), the leakage field will not be caused and the flaw will not be detected. Therefore, certain standards, such as a field sufficient to lift a plate of a particular weight have been used to determine the magnetic field strength necessary for a successful test procedure. Unfortunately, this is an indirect method of testing field strength and cannot be monitored during the course of surface inspection. Since electromagnets are generally used in this test, it is common to calibrate the current used to generate the magnetic field in accordance with the standard and then monitor the current applied to the yoke of the electromagnet. This, however, is also an indirect indicator of magnetic field strength and reliable surface inspection then requires such recalibration to be frequently performed, slowing and complicating the inspection process. Even this procedure does not provide a direct indication of magnetic field strength or provide for continuous monitoring of the field strength during the inspection process.
Further, the magnetic field strength within a body of material may be greatly affected by the geometry through which the magnetic field is applied to the material. Consider for example that the surface roughness of a structural shape or plate will often be greater than the surface flaws to be detected. Thus the surface roughness presents two partial gaps in the magnetic circuit when the electromagnet yoke is applied to the surface. Therefore, the magnetic field may be diminished within the material to be inspected due to the reluctance of the partial gaps in the magnetic circuit. Thus, even frequent calibration in accordance with a standard does not guarantee that an inspection is reliable or that a sufficiently great magnetic field will be generated within the material to cause leakage fields at surface flaws.